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EARTHY MINERALS. Under this head are included such minerals as consist of one earth or more, together with such occasional and variable pro portions of iron, manganese, water, and even of acids, as in duce the conclusion that they are only accessory constituents. We shall begin with silica in its purest form, as being the old est and most abundant mineral, and as affording the most simple arrangement; and then proceed to such as, by the most au thentic analyses, appear to consist chiefly of silica. Those of which alumina forms the greatest proportion succeed, as being the earth next in age and abundance. Magnesia follows ; then such minerals as consist primarily or in part of zirconia, or glu- cina, or lastly of yttria. QUARTZ. Quarz, IV. and II. Rhombohedral Quartz, M. Pure silica accidentally mixed with minute proportions of me tallic oxides,—from whence the fine colours of this species are derived. Rock Crystal. Amethyst. Silica 96-375 97-50 with a trace of ferriferous alumina Oxide of iron 0-75 . and manganese. Bucholz. Alumina 0-25 Rose. Sp. Gr. 2-6. H. = 7-0. Of quartz there are many varieties, most of which the older mineralogists described as distinct species. Some of these differ considerably in their external characters; others nearly agree : they are all sufficiently hard to scratch glass, and, when compact enough, they give fire with steel; they do not yield to the knife; and alone are infusible before the blowpipe, by exposure to which the coloured varieties generally lose their colour. Quartz occurs massive and crystallized; also stalactitic, pseu- domorphous, spongiform, granular, compact, &c. A